A device such as a high-power electric motor, a charger is disposed in an electric vehicle that is one of electric transportation equipment such an EV (Electric Vehicle), a PHEV (Plug-in Hybrid Electric Vehicle) which spreads rapidly in recent years, and such a device is provided with an electronic component which withstands a high voltage and a large current. The electronic component includes a coil and a magnetic core as a basic configuration, and the magnetic core is comprised of a magnetic material such as a MnZn-based ferrite.
In such an application, various mechanical and electric loading conditions occur in the electronic component during running, and a used environmental temperature is also various. In the electronic component used in a consumer-electronics device, a MnZn-based ferrite is used whose composition is designed so that a minimum temperature of a magnetic core loss (also called a power loss) is 100° C. or lower for example, however, it is often the case that the MnZn-based ferrite is used that has the minimum temperature of the magnetic core loss Pcv reaching a high temperature over 100° C., based on the assumption that the MnZn-based ferrite is used under the high-temperature environment for on-vehicle use. Moreover, a low magnetic core loss is required in a wide temperature range.
Generally, the magnetic core loss Pcv of the ferrite consists of a hysteresis loss Ph, an eddy current loss Pe and a residual loss Pr. The hysteresis loss Ph increases in proportion to a frequency due to a direct-current hysteresis, and the eddy current loss Pe increases in proportion to the square of a frequency due to an electromotive force which is generated by an eddy current occurring according to an electromagnetic induction action. The residual loss Pr is the remaining loss which is related to a factor such as a domain wall resonance, and is revealed on a frequency of 500 kHz or more. That is, the hysteresis loss Ph, the eddy current loss Pe and the residual loss Pr change depending on a frequency, and a ratio thereof accounting for the whole magnetic core loss also changes depending on a frequency band.
The magnetic core loss of the MnZn-based ferrite has a temperature dependence, has the low hysteresis loss at a temperature where a crystal magnetic anisotropy constant K1 is zero, and has a minimum value at that temperature. An initial permeability pi is the maximum at that temperature, therefore, it is also called the secondary peak of the initial permeability pi. Since the magnetic core loss has a minimum value concerning the temperature, usually, a temperature at which the magnetic core loss is the minimum is adjusted with the crystal magnetic anisotropy constant K1 in anticipation of the generation of heat by the magnetic core loss, and the temperature is set to a temperature slightly higher than an environmental temperature to which the electronic component is exposed, which prevents the ferrite from losing magnetism due to thermal run-away.
The temperature at which the magnetic core loss is the minimum, i.e., the temperature at which the crystal magnetic anisotropy constant K1 is zero, can be changed according to the sum obtained by appropriately adjusting an amount of a metal ion having a positive crystal magnetic anisotropy constant K1 and an amount of a metal ion having a negative crystal magnetic anisotropy constant K1 among metal ions mainly constituting spinel in the MnZn-based ferrite. For the metal ions constituting spinel, the metal ions having the positive K1 are Fe2+ and Co2+ and the like and the metal ions having the negative K1 are Fe3+, Mn2+, Ni2+, and the like. Although the change of the temperature at which the magnetic core loss is the minimum can be comparatively easy by adjusting the metal ions such as Fe2+, Fe3+, Zn2+, and Mn2+, it is difficult to improve the temperature dependence of the magnetic core loss based on such a process only. Thus, Co2+ is employed that has a crystal magnetic anisotropy constant and a magnetostriction constant adequately larger than those of Fe2+, which improves the temperature dependence of the magnetic core loss.
Japanese Patent Laid-Open Publication No. 2001-220146 discloses a MnZn-based ferrite which contains Fe2O3: 52.0-55.0 mol %, MnO: 32.0-44.0 mol % and ZnO: 4.0-14.0 mol % as a main component and contains CaO: 200-1000 ppm, SiO2: 50-200 ppm, Bi2O3: 500 ppm or less, Ta2O5: 200-800 ppm and CoO: 4000 ppm or less as a sub component. In the MnZn-based ferrite disclosed in Japanese Patent Laid-Open Publication No. 2001-220146, the balance of the metal ions is adjusted with a composition amount of Fe2O3, CoO, ZnO, MnO, etc., and the temperature at which the magnetic core loss is the minimum is changed, which improves the temperature dependence of the magnetic core loss, while Bi2O3 is added to obtain a MnZn-based ferrite whose magnetic core loss is low in a wider temperature range.
Such an employment of Co2+ is effective in the improvement of the temperature dependence of the magnetic core loss. However, the divalent metal ion such as Fe2+ and Co2+ is easy to move via a lattice defect to cause the increase of magnetic anisotropy, and also time-dependent change of a magnetic property such as the increase of the magnetic core loss and the decline of a magnetic permeability. Especially, it is known that the MnZn-based ferrite containing Co has such a marked tendency and the time-dependent change is accelerated under the high-temperature environment. Accordingly, the MnZn-based ferrite used for an electronic component which is easy to be exposed to a high temperature is required to further lower the magnetic core loss and to suppress control the time-dependent change of the magnetic property.
As a method of suppressing the time-dependent change of the magnetic property of the MnZn-based ferrite, Japanese Patent Laid-Open Publication No. 2004-292303 and Japanese Patent Laid-Open Publication No. 2007-70209 disclose to control an ambient oxygen concentration in calcination. The calcination includes a temperature rising step, a high temperature maintaining step and a temperature falling step as a basic process, and in Japanese Patent Laid-Open Publication No. 2004-292303 and Japanese Patent Laid-Open Publication No. 2007-70209 the ambient oxygen concentration is strictly controlled at the high temperature maintaining step and the temperature falling step especially.